It comes after Anis Amri, a 24-year-old Tunisian suspected of carrying out the truck attack which killed 12 people, was shot dead in a town near Milan early on Friday after he pulled a gun on police during a routine check.

In France, where Islamist militants killed 130 people in shooting and bomb attacks in Paris in November 2015, authorities said more than 91,000 policemen and soldiers would be deployed, with additional security measures being enforced at churches.

Meanwhile, armed guards patrolled the area around a church as Christians attended a Christmas Eve service in Bartalla, near Mosul, Iraq.

Pope Francis kisses a statue of baby Jesus as he leads the Christmas night Mass in Saint Peter's Basilica at the Vatican

Police there have increased security at a number of popular tourist attractions, including around Queen Elizabeth's London residence, Buckingham Palace.

Rome authorities have banned vans or trucks from entering the city centre and anti-terror police wearing masks and wielding machine guns set up roadblocks on routes leading to famous tourist sites or areas where crowds traditionally gather.

At the Vatican, where Pope Francis was due to celebrate Christmas Eve Mass in St. Peter's basilica on Saturday evening, police cars and military jeeps stood about every 100 metres along streets leading to the Vatican.

Security was also stepped up in central Milan and other Italian cities, particularly near major churches where the faithful were attending Christmas services.

Turkish police check the grounds of the Saint Anthony of Padua Church prior to the start of the Christmas Eve mass in Istanbul

A Turkish security officer checks the purse of a woman a she enters Saint Anthony of Padua Church prior to the start of the Christmas Eve mass

A Pakistani security officer stands guard outside the Saint Anthony's church during a midnight service on Christmas eve in Lahore, Pakistan

Children were photographed trying to grab ornaments from a tree in the old city of Damascus, Syria, on Christmas Eve

People took selfie photos near a Christmas tree outside the state-approved Xuanwumen Catholic Church, otherwise known as the South Cathedral in Beijing